Barrack Obama President of the United States since .A. became aB. has become aC. has

问题补充：

Barrack Obama President of the United States since .

A. became a

B. has become a

C. has turned

D. has been

答案：

A. became a

B. has become a

C. has turned

(答案→)D. has been

解析：Barrack Obama自从一直是美国的总统。和since 连用，用现在完成时，而且因为是一段时间，所以不能和短暂动词become连用，所以选D。